1. Field of the Invention
The present invention relates in general to a fluid-filled resilient or elastic bushing structure, and more particularly to such a bushing structure which is capable of exhibiting excellent spring characteristics for both low-frequency vibrations and high-frequency vibrations that are applied thereto in a diametrical direction of the structure.
2. Discussion of the Prior Art
There is known a resilient bushing for elastically connecting two members in a vibration system (through which vibrations are transmitted), for damping and/or isolating vibrations applied to the bushing in a given diametric direction of the bushing. The bushing has an inner sleeve in which a mounting rod or bolt is inserted, an outer sleeve on which a cylindrical mounting member is fitted, and a resilient member interposed between the inner and outer sleeves. For example, such a resilient bushing is used as a suspension bushing in a suspension system of an automotive vehicle, or an engine mount for mounting a power unit on the body of an F-F vehicle (front-engine, front drive vehicle).
Usually, the resilient bushing of the type indicated above is required to exhibit high vibration isolating characteristic for high-frequency vibrations having a small amplitude, and high vibration damping characteristic for low-frequency vibrations having a large amplitude. The traditional resilient bushing relies solely on the elastic nature (elastic deformation) of a resilient or elastic member, to provide both the vibration isolating capability and the vibration damping capability. Therefore, the bushing is difficult to satisfy these two different requirements. In particular, the traditional resilient bushing is not satisfactory in its capability of damping the low-frequency vibrations of large amplitudes.
In the light of the above inconvenience, a fluid-filled resilient bushing has been proposed in recent years. An example of such a fluid-filled bushing is disclosed in U.S. Pat. Nos. 3,642,268 and 3,698,703. This fluid-filled bushing has a pair of fluid chambers formed in a resilient member such that the fluid chambers are located opposite to each other in a diametric direction of the bushing in which vibrations are applied. These fluid chambers are filled with a suitable incompressible fluid, and communicate with each other through an orifice, so that the fluid may flow through the orifice, between the two chambers, upon application of low-frequency vibrations of a large amplitude in the diametric direction of the bushing.
In the fluid-filled bushing indicated above, the input low-frequency vibrations can be effectively damped due to inertia and resonance of the fluid mass in the orifice. The frequency range of the vibrations to be damped can be selected by suitably dimensioning the orifice.
If the orifice of this type of fluid-filled resilient bushing is dimensioned (in terms of its length and cross sectional area or diameter) so as to provide excellent damping capability for vibrations in a low frequency range, then the vibration isolating capability of the bushing is accordingly reduced for the high-frequency vibrations. The conventional bushings having a pair of diametrically opposite fluid chambers can not effectively isolate vibrations having a frequency higher than the resonance point of the fluid mass in the orifice, since the fluid mass in each fluid chamber becomes too rigid and the dynamic spring constant of the bushing becomes too high when such high-frequency vibrations applied to the bushing. Thus, there has been a need to develop a fluid-filled resilient bushing which is satisfactory in the overall vibration damping and isolating capability.
The present inventor has proposed in Japanese patent application No. 59-267768, filed Dec. 19, 1984, a fluid-filled resilient bushing of a type which has a pressure-receiving chamber adapted to receive axial vibrations to be damped, and an equilibrium chamber partially defined by an elastically yieldable thin-walled partition member. The pressure-receiving chamber and the equilibrium chamber communicate with each other through an orifice, and elastic deformation of the partition member permits a change in the volume of the equilibrium chamber. In this arrangement, the volume of the pressure-receiving chamber can be changed with flows of the fluid between the two chambers through the orifice, accompanied by elastic deformation of the partition member of the equilibrium chamber. This type of fluid-filled bushing provides excellent damping characteristics based on the flow resistance of the orifice and the inertia of the fluid masses, for low-frequency vibrations having a large amplitude. However, like the bushing disclosed in the United States Patents identified above, the bushing having such pressure-receiving and equilibrium chambers fails to provide satisfactory overall damping and isolating capability for both low-frequency and high-frequency vibrations of different amplitudes.
Another type of fluid-filled bushing has been proposed, as disclosed in U.S. Pat. Nos. 4,159,091 and 4,422,779. This bushing employs a pressure-absorber mechanism incorporating a movable plate which is disposed between two fluid chambers, so as to partially define these chambers. The movable plate is adapted to be moved by changing pressures in the two chambers, thereby contributing to lowering the dynamic spring constant of the bushing, for effectively isolating high-frequency vibrations of a small amplitude. However, the bushing using such a pressure-absorber mechanism tends to be extremely complicated in construction and accordingly less economical to manufacture, and requires a relatively large space for the pressure-absorber mechanism, causing the bushing structure to be bulky.